1. Technical Field
The present invention relates to a control channel placement method in which control channels for establishing wireless communication between a base station and a terminal station in a wireless communication system are placed in optimal time slots within a TDMA-TDD frame. The terminal station could be a vehicle telephone, a portable telephone or the like.
2. Discussion of the Background
A cellular system is generally used for providing a communication service for vehicle telephones or portable telephones. This cellular system comprises a plurality of base stations and small zones (referred to hereafter as cells) around each base station. The cells are so constructed that there is no gap between then.
In the cellular system, same channel is repeatedly used in zones under the condition that no mutual interference obstruction is generated. Therefore, it is possible provides a service in a very wide area with less number of channels. When the antenna of the base station is non-directional within the horizontal plane, and if the effects of topography and ground based obstructions are ignored, then the shape of the area covered by each base station will be circular. These circular cells are called omnicells. Switching of channels is performed in the portions where adjacent omnicells overlap.
On the other hand, sector cell structure zones are also known. In case of sector cell structure zones, directional antennas are provided in each base station. The beam width of each antenna is set so as to divide a circular area into a plurality of fan-shaped cells (referred to hereafter as sector cells) each having the same size.
Since directional antennas are used, it is possible to remove the effect of interference waves that arrive from directions other than the direction to which the antenna is pointed. In addition, there is no possibility of interfering waves broadcast towards base stations using the same channel but situated in a direction other than the one to which the antenna is pointed. As a result, it is possible to decrease the distance between base stations to which the same channel has been allocated and to improve the spatial channel usage efficiency. Moreover, the overall channel usage efficiency is improved in comparison with omnicells using non-directional antennas.
Because of reasons such as this, in wireless communication systems for performing mobile communication such as vehicle telephones and portable telephones, as the number of users increases, there is currently a transition underway from employing a zone structure that uses omnicells having a small radius to employing a zone structure that uses sector cells.
In the meanwhile, time division multiple access (TDMA) mode is known as the representative mode among digital wireless access modes employed in vehicle telephones and portable telephones. In TDMA mode, signals having a predetermined frequency bandwidth are divided along a time axis and communication is performed by cyclically allocating the divided time band to each user as one channel. Using this TDMA mode, a user can perform communication in time slots allocated into short cycles in the same frequency having a predetermined bandwidth.
The characteristics of this TDMA mode are described below. Firstly, a plurality of users can perform communication simultaneously using a single transreceiving device, therefore, the base station can be constructed with a small size and at a low cost. Secondly, open time is generated because the communication time only needs to be the time of the allocated time slot, thus allowing the state of peripheral bands to be observed at this time. Thirdly, because the operating time during communication is intermittent, the power consumption in the terminal station can be reduced.
Furthermore, when the TDMA mode is employed as a wireless access mode, it is possible to use the time division duplex (TDD) mode in combination with the TDMA mode. The TDD mode uses the same frequency for both, reception and transmission, between a terminal station and a base station and performs the transmission and reception by dividing the time. In the TDD mode, the characteristics of the wave propagation are the same going xe2x80x9cupxe2x80x9d (i.e. from the terminal station to the base station) as they are going xe2x80x9cdownxe2x80x9d (i.e. from the base station to the terminal station). Therefore, it is possible to only have to carry out countermeasures against variations in the wave propagation at the base station side only.
The frame structure of the transmission and reception using this TDMA-TDD may be one structured, for example, from four pairs of time slots (one up and one down). In this case, the first pair is used as a control channel with the remaining three pairs being used as notification channels. As a result, in this example, three terminal stations can perform communication with one base station at the same time.
In this case, it is possible to divide the control channels into xe2x80x9cdown control channelsxe2x80x9d going in the direction from the base station to the terminal station and xe2x80x9cup control channelsxe2x80x9d going in the direction from the terminal station to the base station. The down control channels are formed from a broadcast channel (BCCH) for notifying a terminal station as to the ID of the host base station and about information peculiar to the base station such as the control channel frequencies of peripheral base stations and the like, and from a paging channel (PCH) for transmitting incoming message information, and the like.
The up control channels are formed from a random access type of signal control channel (SCCH) for performing tasks such as receiving transmissions and specifying wireless channels. FIG. 14 is an explanatory diagram showing the frame structure in a conventional TDMA-TDD mode. In FIG. 14, in accordance with the above example, four pairs of time slots comprising up and down are set as one TDMA-TDD frame.
In particular, in this example, the up control channel is a multiframe structure in which 20 frames are set as one new frame, while the down control channel is a super frame structure in which 12 multiframes are set as one new frame.
The TDMA-TDD mode has commonly been applied as a wireless access mode in portable telephones and vehicle telephones. Various control channel setting methods which should be able to realize higher quality communication are provided in a wireless communication system that employs this TDMA-TDD mode.
For example, according to the xe2x80x9cControl Channel Setting Methodxe2x80x9d disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-245780, in order to reduce interference in its own down control channel, a parent station (a base station) measures the reception levels of all channels of frequencies used for control signals (control channels), and selects, for setting as control channels, usable combinations from the reception levels of each channel from among combinations of up control channels and down control channels that form pairs in the TDMA-TDD mode, such that up control channels and down control channels of peripheral base stations do not overlap.
As another example, according to the xe2x80x9cRadio Control Systemxe2x80x9d explained in U.S. patent application No. 09/236,392, by the same assignee as the present application in the wireless communication system of a sector cell zone structure that uses directional antennas, when a particular base station performs communication, interference is reduced by not using antennas transmitting in the same direction as the antennas used for communication by base stations adjacent to that particular base station, based on tables that have been prepared in advance.
Moreover, as the channel allocation method, a fixed channel allocation method may be used in which the channels to be used by each cell are fixed in advance with consideration given to mutual interference between zones. Alternatively, a dynamic channel allocation method in which the channel allocation is changed time-wise in accordance with the demands of the calls in each cell may be used.
In the channel allocation method, in particular, a specific channel that can be used by the base station is selected for each communication from among all the channels. The carrier to interference (CI) wave strength ratio is then measured for the selected channel thereby allowing the reception levels of the carrier and interference waves to be detected. A determination is then made as to whether or not that channel can be allocated.
In the dynamic channel allocation method it is possible to respond flexibly in the channel allocation to variations in the voice level that occur overtime. Therefore, the efficiency of the utilization of the frequencies is better in the dynamic channel allocation method than in the fixed channel allocation method. Accordingly, in a wireless communication system, by combining a dynamic channel allocation method with the sector cell zone structure, a remarkable improvement in the efficiency of the utilization of the frequencies can be achieved.
However, the xe2x80x9cControl Channel Setting Methodxe2x80x9d disclosed in JP-A No. 7-245780 is intended to reduce interference in control channels between a terminal station and a plurality of base stations in an omnicell zone structure, and it is not able to reduce interference in control channels allocated to each sector for a base station having a sector cell structure.
Furthermore, although the xe2x80x9cRadio Communication Systemxe2x80x9d according to the above-mentioned U.S. patent application No. 09/236,392 is designed to reduce interference between adjacent base stations, in order to achieve that, it does not alter the placement of the control channels, and is not able to solve interference problems when communication is performed simultaneously by a plurality of terminal stations.
Moreover, when the dynamic channel allocation method is used, if the carrier to interference ratio of a candidate allocation channel is above a predetermined threshold value, that channel will be allocated, however, the possibility will exist that the allocation will generate interference obstruction in surrounding terminal stations or base stations that are communicating using the same channel. In addition, if the base stations have a sector cell structure, because the amount of interference received by each cell is changed by differences in the timing of the channel allocation, it has not been possible to obtain a constantly high quality communication state.
In particular, in a sector cell structure using directional antennas having a directionality of 60xc2x0 within a regular hexagonal shaped cell, namely, in a base station having 6 sector cells, because the sector cells are opposite each other, cases arise in which the direction of waves transmitted to a terminal station by one sector cell match the direction of waves received from the terminal station by the sector cell in the opposite position, thereby generating interference between the two.
Moreover, when it is only possible to place one control channel in one cycle (the number of TDMA-TDD frames extending for the number of sectors), there are times when it is not possible to place control channels at fixed time slot positions inside the TDMA-TDD frame of one cycle. In this case, interference and the like is generated after communication is established, and when reception in the control channel becomes impossible, it is necessary to reallocate the TDMA-TDD frames.
It is an object of the present invention to provide a method for placing control channels which sets high quality wireless channels in a wireless communication system employing the TDMA-TDD mode by placing control channels in suitable time slots so that interference from an identical frequency can be avoided.
According to the control channel placement method of the present invention, in a wireless communication system in which a base station having directional antennas that match placement angles of a plurality of sector cells performs communication in a TDMA-TDD mode with a plurality of terminal stations, up and down control channels for establishing communication by the base station with the terminal stations in each sector cell are placed in predetermined time slot positions within TDMA-TDD frames, comprising: a reception level measurement step in which a reception level of each time slot extending over TDMA-TDD frames corresponding to the number of sector cells of the base station is measured in each of the sector cells; a time slot extraction step in which time slots in which control channels can be placed are extracted from among the time slots extending over the TDMA-TDD frames corresponding to the number of sector cells, using a result of the measurement of the reception level in the reception level measurement step when a first sector cell was measured, and a result of the measurement of the reception level in the reception level measurement step when a second sector cell in a position opposite the first sector cell was measured from among the sector cells; and a frame allocation step in which the TDMA-TDD frames are allocated for each sector cell such that those predetermined time slot positions in which control channels are placed, from among the time slots within the TDMA-TDD frames, are matched with time slots extracted in the time slot extraction step.
According to the above invention, in a wireless communication system employing the TDMA-TDD mode as a wireless access mode and employing sector cells as the zone structure, when a base station having a plurality of sector cells performs a transmission with both up control channels and down control channels placed in predetermined time slot positions within the TDMA-TDD frames for each sector cell, the reception level in each time slot extending across the TDMA-TDD frames corresponding to the number of sector cells of the base station is measured in the reception level measurement step. Next, in the time slot extraction step, time slots in which it is possible for control channels to be placed are extracted from among the time slots extending across the TDMA-TDD frames corresponding to the number of sector cells, using the result of the measurement of the reception level when the first sector cell was measured, and the result of the measurement of the reception level when the second sector cell at a position opposite the first sector cell was measured from among all the sector cells in the same base station. Finally, in the frame allocation step, the TDMA-TDD frames are allocated for each sector cell such that those predetermined time slot positions in which control channels are placed, from among the time slots within the TDMA-TDD frames, are matched with the time slots extracted in the time slot extraction step. As a result, both the up control channels and the down control channels can be placed simultaneously in optimum time slot positions.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, in the time slot extraction step, a time slot in which a reception level measured in the first sector cell in the reception level measurement step is equal to or less than a predetermined value, and in which a reception level measured in the second sector cell in the reception level measurement step is equal to or less than a predetermined value, is determined to be a time slot in which the control channel can be placed in the first sector cell.
According to the above invention, the time slot extraction step specifies those time slots, from among the time slots within TDMA-TDD frames extending for the number of sector cells, whose reception level in a first sector cell measured in the reception level measurement step is a predetermined threshold value or less. The time slot extraction step then determines that, out of the specified time slots, those time slots whose measurement result in a second sector cell (i.e. the sector cell located opposite the first sector cell) measured in the reception level measurement step is a predetermined threshold value or less are time slots in which control channels can be placed in the first sector cell. As a result, it is possible to remove time slot positions that are in an unstable state due to interference and the like from being candidates for the placement of an up control channel or a down control channel.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, in the time slot extraction step, time slots in which control channels can be placed in the first sector cell are determined in sequence starting from the time slot in which a value, determined by combining a reception level obtained in the reception level measurement step by measuring the first sector cell with a reception level obtained in the reception level measurement step by measuring the second sector cell, is at the minimum.
According to the above invention, the time slot extraction step adds the result of measuring the reception level in the second sector cell (i.e. the sector cell located opposite the first sector cell) to the result of measuring the reception level in the first sector cell measured in the reception level measurement step, and determines the time slots in which control channels can be placed in the first sector cell in sequence starting from that time slot in which the value obtained from the above addition is at the minimum. Therefore, it is possible to allocate time slot positions giving a more stable state of communication as the positions for placing up control channels and down control channels.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, the TDMA-TDD frames are formed by securing a plurality of time slot positions in which up control channels can be placed and a plurality of time slot positions in which down control channels can be placed and by placing in the plurality of time slot positions control channels of sector cells that are different from each other, and wherein, in the frame allocation step, within TDMA-TDD frames extending for the number of the sector cells, specific TDMA-TDD frames are allocated such that time slots extracted in the time slot extraction step coincide with either one of time slot positions for the placement of the control channels within the specific TDMA-TDD frames determined for each of the sector cells and time slot positions for the placement of the control channels within other TDMA-TDD frames different to the specific TDMA-TDD frames.
According to the above invention, the TDMA-TDD frames are formed by securing a plurality of time slots positions for the respective placement of up control channels and down control channels and by placing control channels for sector cells that are different from each other in the plurality of time slot positions. When the frame allocation step allocates specific TDMA-TDD frames (namely, TDMA-TDD frames corresponding to specific sector cells), there is a prerequisite that, within TDMA-TDD frames extending for the number of the sector cells, time slots extracted in the time slot extraction step coincide with one of either time slot positions for the placement of the above control channels within the specific TDMA-TDD frames and time slot positions for the placement of the above control channels within other TDMA-TDD frames different to the specific TDMA-TDD frames. Therefore, it is possible to choose from a plurality of control channel placement positions spread across the number of TDMA-TDD frames corresponding to the number of sector cells in order to allocate TDMA-TDD frames for one sector cell.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, the TDMA-TDD frames are formed by securing a plurality of time slot positions in which up control channels can be placed and a plurality of time slot positions in which down control channels can be placed and, based on a reception level measurement result measured in the reception level measurement step, control channels of sector cells that are different from each other are placed in a plurality of time slot positions for the placement of the control channels, and wherein, in the frame allocation step, within TDMA-TDD frames extending for the number of the sector cells, specific TDMA-TDD frames are allocated such that time slots extracted in the time slot extraction step coincide with either one of time slot positions for the placement of the control channels within the specific TDMA-TDD frames determined for each of the sector cells and time slot positions for the placement of the control channels within other TDMA-TDD frames different to the specific TDMA-TDD frames.
According to the above invention, the TDMA-TDD frames are formed by securing a plurality of time slot positions for the placement of up control channels and down control channels and, based on a reception level measurement result measured in the reception level measurement step, by placing control channels for sector cells that are different from each other in each of the plurality of time slot positions. When the frame allocation step allocates specific TDMA-TDD frames (namely, TDMA-TDD frames corresponding to specific sector cells), there is a prerequisite that, within TDMA-TDD frames extending for the number of the sector cells, time slots extracted in the time slot extraction step coincide with one of either time slot positions for the placement of the above control channels within the specific TDMA-TDD frames and time slot positions for the placement of the above control channels within other TDMA-TDD frames different to the specific TDMA-TDD frames. Therefore, it is possible to choose from a plurality of control channel placement positions spread across the number of TDMA-TDD frames corresponding to the number of sector cells in order to allocate TDMA-TDD frames for one sector cell.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, there is included a reception level remeasurement step for measuring, at regular intervals and for each of the sector cells, reception levels of time slots in which control channels have been placed in the frame allocation step; and a frame reallocation step for performing the frame allocation step when a reception level measured in the reception level remeasurement step is at a predetermined threshold level or greater.
According to the above invention, in a wireless communication system that employs a TDMA-TDD mode for the wireless access mode and employs sector cells for the zone structure, when a base station having a plurality of sector cells performs transmission by placing both up control channels and down control channels at predetermined time slot positions within the TDMA-TDD frames in each sector cell, the reception level in each time slot extending for the number of TDMA-TDD frames corresponding to the number of sector cells of the base station is measured in the reception level measurement step. Next, in the time slot extraction step, time slots in which control channels can be placed are extracted from among the time slots that extend across TDMA-TDD frames of the number of sector cells using the result of the measurement of the reception levels measured in a first sector cell and the result of the measurement of the reception levels measured in a second sector cell that is located opposite the first sector cell from among all the sector cells in the same base station. Next, with communication having been established as a result of TDMA-TDD frames allocated in each sector cell in the frame allocation step such that time slots extracted in the time slot extraction step coincide with predetermined time slot positions for the placement of control channels from among the time slots within the TDMA-TDD frames, the reception level remeasurement step measures at regular intervals the reception levels of time slots in which control channels have been placed in each sector cell. If the result of this reception level measurement is equal to or greater than a predetermined threshold, the frame reallocation step performs the frame allocation step once again. As a result of the above, it is possible to avoid deterioration in the quality of communication caused by interference obstruction generated while communication is established.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, the frame allocation step performed in the frame reallocation step allocates the TDMA-TDD frame to a position shifted by one time slot towards the front or rear.
According to the above invention, because the above frame allocation step performed in the frame reallocation step allocates the TDMA-TDD frame to a position shifted by one time slot towards the front or rear, by attempting to establish communication each time by shifting the reallocation of the TDMA-TDD frames by one time slot forwards or backwards, it is possible to omit the calculation to search for time slots in which control channels can be placed. At the same time, it is possible to keep the time when communication cannot be established to the minimum.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, in the reception level remeasurement step, the carrier to interference wave strength ratio is measured from the reception levels and, in the frame reallocation step, the frame reallocation step is performed when the carrier to interference wave strength ratio measured in the reception level remeasurement step is equal to or greater than a predetermined threshold.
According to the above invention, because the carrier to interference wave strength ratio is acquired from the reception levels measured in the reception level remeasurement step, and the frame reallocation step is able to perform frame reallocation in accordance with this carrier to interference wave strength ratio, it is possible to accurately determine whether or not a control channel is receiving interference obstruction.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, in the reception level remeasurement step, the carrier to interference wave strength ratio is measured from the reception level when the time slot being measured is in a blocked state and, in the frame reallocation step, the frame reallocation step is performed when the carrier to interference wave strength ratio measured in the reception level remeasurement step is equal to or greater than a predetermined threshold.
According to the above invention, because the carrier to interference wave strength ratio is acquired from the reception levels measured in the reception level remeasurement step when the time slots being measured are in a blocked state, and the frame reallocation step is able to perform frame reallocation in accordance with this carrier to interference wave strength ratio, it is possible to accurately determine whether or not variations in the reception level in a control channel are only caused by interference obstruction.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, in the frame reallocation step, if the carrier to interference wave strength ratio measured in the reception level remeasurement step continues to be equal to or greater than a predetermined threshold for a predetermined length of time, then a determination is made that the time slot being measured is receiving interference obstruction and the frame allocation step is performed.
According to the above invention, because the frame reallocation step is able to determine that the time slot being measured is receiving interference obstruction if the measured carrier to interference wave strength ratio continues to be equal to or greater than a predetermined threshold for a predetermined length of time, and then perform the frame allocation step, it is possible to accurately determine whether variations in the reception level are caused by collisions in the up control channels or by interference obstruction.
Moreover, according to the control channel placement method of the present invention, in the above control channel placement method, in the reception level remeasurement step, an error detection ratio in an up control channel is measured as a reception level and, in the frame reallocation step, if the error detection ratio measured in the reception level remeasurement step is equal to or greater than a predetermined threshold, the frame allocation step is performed.
According to the above invention, because the error detection ratio in the up control channel can be measured in the reception level remeasurement step and the frame reallocation step can perform the frame allocation step in accordance with this error detection ratio, it is possible to determine whether the usage of the time slot of a control channel has become difficult or impossible.